(1) Field of the Invention
This invention relates to an exposure mask and a pattern exposure method, and more particularly to an exposure mask for use in forming a pattern of wiring and pixel electrodes on a thin film transistor (TFT) substrate by step exposure, and a pattern exposure method for exposing the TFT substrate.
(2) Description of the Related Art
On a TFT substrate used in a liquid crystal display device, there are formed numerous TFTs corresponding in number to the number of pixels. FIG. 6 is a plan view showing an essential portion of a TFT substrate by way of example. The TFT substrate 100 shown in FIG. 6 is an active matrix substrate having pixels 101 arranged in matrix, and on the TFT substrate 100, each of the TFTs 102 formed for respective pixels 101 is connected to pixel electrodes 103 and wiring 104 including a gate bus line 104a and a drain bus line 104b. A whole pattern of pixel electrodes 103 and wiring 104 formed on such a TFT substrate includes lots of parts formed as repetitions of an identical pattern. Therefore, the pattern of pixel electrodes 103 and wiring 104 can be efficiently formed by step exposure in which an area containing a plurality of pixels 101 is exposed by one shot, and other areas having the same pattern as formed by the one shot are similarly repeatedly exposed.
However, in the case of forming a pattern on a TFT substrate by the step exposure described above, if positional deviation of an exposure area occurs during a shot, the deviation causes differences in the amount of exposure light and the position of exposure between pixels in a joint portion between patterns formed by the step exposure (hereinafter referred to as “step-exposure patterns”) and the other pixels outside the joint portion. The difference in the amount of exposure light and the position of exposure shows up as deviation of dimensions of the wiring and the pixel electrodes from design dimensions, which causes difference in brightness between the formed pixels. As a result, on a liquid crystal display device using the TFT substrate, the joint portions of the step-exposure patterns appear as portions exhibiting display non-uniformity. As the size of a TFT substrate becomes larger, the step exposure produces a larger number of such joint portions. Therefore, it is crucially important to reduce the display non-uniformity.
Recently, a solution to this problem has been proposed in which the step exposure is performed using an exposure mask having a mask pattern formed such that shield portions corresponding to respective pixels are mosaically arranged in end portions of the mask pattern, so as to avoid occurrence of display non-uniformity (see e.g. Japanese Unexamined Patent Publication No. H09-236930).
FIGS. 7A and 7B are plan views schematically showing essential portions of a conventional exposure mask by way of example. FIG. 7A shows a mask pattern formed in a one-end portion of the exposure mask, and FIG. 7B shows a mask pattern formed in the other-end portion of the same. The right-side end of FIG. 7A corresponds to one end of the exposure mask, while the left-side end of FIG. 7B corresponds to the other end of the same. As shown in FIG. 7A, the mask pattern 110a formed in the one-end portion of the exposure mask has a mosaic arrangement of pattern-forming portions 111 each formed with traces to be exposed for one pixel and shield portions 112 each for blocking transmission of exposure light for one pixel. As shown in FIG. 7B, in the mask pattern 10b formed in the other-end portion of the exposure mask, the positional relationship between the pattern-forming portions 111 and the shield portions 112 is reverse to that in FIG. 7A. In short, the exposure mask has the mask patterns 110a and 110b formed in the respective opposite end portions in a manner complementary to each other.
FIG. 8 is a view useful in explaining the step exposure. In the step exposure carried out using the exposure mask shown in FIGS. 7A AND 7B, for example, respective end portions of step-exposure patterns 121 and 122 to be formed laterally adjacent to each other on a substrate 120 by respective shots are exposed in an overlapping manner (overlap exposure). The overlapping area is not only exposed through the mask pattern 110a, shown in FIG. 7A, in the one-end portion of the exposure mask by a shot I for forming the step-exposure pattern 121, but also exposed through the mask pattern 110b, shown in FIG. 7B, in the other-end portion by a shot II for forming the step-exposure pattern 122.
The conventional step exposure is carried out, as described above, by using the exposure mask having the mask patterns in the opposite end portions thereof formed in a manner complementary to each other, to thereby reduce display non-uniformity in the joint area where the step-exposure patterns overlap each other.
However, the conventional exposure mask suffers from the following several problems:
First, in the exposure mask having the shield portions each formed to have approximately the same size as the pattern-forming portion, when positional deviation occurs during step exposure, an area shielded during a preceding shot and an area shielded during the following shot often overlap each other to cause non-exposure in the area. To solve the problem, each shield portion is required to be formed to have a slightly smaller size than the pattern-forming portion. For this reason, the exposure mask has the shield portions each of which corresponds to one pixel and has a slightly smaller size than the pattern-forming portion, as shown in FIGS. 7A AND 7B. In this case, a gap is created between shield portions adjacent to each other.
The gap between each two of the adjacent shield portions allows exposure light to leak therethrough during step exposure.
An area to be shielded from exposure light by an adjacent shield portion during a following shot is an area exposed during the preceding shot. Therefore, if exposure light leaks through a gap between shield portions during the following shot, the area having already been exposed by the preceding shot is partially exposed again. Thus, in the step exposure carried out using the exposure mask described above, even when an exposure area is not positionally deviated, an area having already exposed can be overexposed by light leaked through a gap between shield portions. When traces, e.g. of a drain bus and pixel electrodes, to be formed near a border area between adjacent pixel areas are exposed once by a preceding shot and thereafter exposed again by the following shot, the line width of the traces can be made smaller than the design dimension. This brings about the difference in brightness between pixels and can be a cause of display non-uniformity of a liquid crystal display device. Variation in the line width of a pattern by overexposure is more conspicuous as the pattern formed on a TFT substrate is finer.
Further, the mosaic areas of the exposure mask for exposure of the joint area of step-exposure patterns cannot prevent occurrence of display non-uniformity only by laying out the pattern-forming portions and the shield portions in a simple mosaic arrangement. To realize an enhanced quality of images displayed by the liquid crystal display device, full consideration must be given to the layout of the mosaic areas.